It is known that the standard interlaced television (TV) signal has a sampled-in-time structure, because each frame is formed by two interlaced consecutive fields, each field being in turn composed of a given number of lines, e.g., even lines or odd lines.
The sampled structure of the TV signal is responsible for some problems. For example, problems known in the art as "raster visibility", "line flickering", "line crawling" are related to the interlaced structure of the TV signal, while problems like "field flickering" are related to the field frequency.
Known techniques for overcoming, or at least reducing these problems without changing the standard of the transmitted TV signal involves performing a digital signal processing at the receiver end.
Such techniques are substantially based on interpolation algorithms, and can be divided in two categories, namely Interlaced Progressive Conversion (IPC) and Field Rate Up-conversion (FRU). In the IPC technique, the number of lines for a field shown on the TV screen is doubled; for example, when an even field is received, odd lines are inserted between the even lines, the odd lines being obtained by interpolating the information content of two or more adjacent even lines. The same happens when an odd field is received. In the FRU technique, the number of fields shown per unit time on the TV screen is doubled with respect to the number of fields received per unit time. The circuits implementing these algorithms are called IPC filters or FRU filters, respectively.
The TV signal has two spatial dimensions, horizontal and vertical, and a time dimension. Taking account of this, both IPC and FRU interpolation algorithms can be further divided in two classes: two-dimension or intra-field algorithms and three-dimension or inter-field algorithms. Intra-field algorithms operate only on the two spatial dimensions of the TV signals, while inter-field algorithms use both the two spatial dimensions and the time dimension of the TV signal. Four classes of filters can thus be individuated: intra-field IPC or FRU filters, and inter-field IPC or FRU filters.
Intra-field and inter-field filters have different performance, but they also have different requirements in terms of practical implementation. For example, an inter-field IPC filter requires a buffer line memory for the current line data and a field memory containing data of the previous field, assuming that only the previous field is used for interpolation. The buffer line memory is necessary because the number of lines to be shown on the TV screen per unit time is twice the number of lines received per unit time. In an inter-field FRU filter, two field memories are necessary, namely a buffer field memory and a field memory for the previous field data. The buffer field memory is necessary because the number of fields shown on the TV screen per unit time is twice the number of received fields per unit time. Intra-field filters require less memory: an intra-field IPC filter does not require any field memory, while an intra-field FRU filter requires only one field memory, i.e., the buffer field memory.
Due to the technological limits of current integrated circuits manufacturing techniques, has not been possible to integrate in a same chip both the interpolation filter and the required memories, particularly when field memories must be provided; therefore, external memory chips must be provided. For example, a chip set for implementing an inter-field interpolation algorithm is formed by the filter chip and at least one field memory chip. These chips obviously increase the costs. Consequently, TV appliance manufacturers must choose the type of filter provided with the TV set in view of the cost, not only of the filter chip, but also of the required memory chips.
Also, commercially available filter chips are either of the intra-field or inter-field type: this increases inventory costs for the integrated circuit manufacturers and reduces design flexibility at the TV appliance manufacturer end.